1. Field of the Invention:
The invention concerns a lettering device with a keyboard for choosing the symbols to be written and a special part which can be separated from the bank of keys if necessary which exhibits a writing pen which can be moved from the rest position to the writing position; which by means of two motors corresponding to the symbols to be drawn, can be moved in two coordinate directions, whereby at least the writing part can be attached to the drawing head of the drafting machine.
2. Brief Description of the Prior Art:
With a known lettering device of this type (DE - PS No. 12 41 312) the writing part is arranged on a guide track and is movable by a threaded spindle driven by one of the motors along this guide track. The writing arm located on the writing part is rotatable by means of another motor around an axis so that the writing pen moves along a circular, arched path. By guiding the writing part along the guide track and by superimposing a swivel movement of the writing arm and, thereby, the writing pen over this movement, the desired symbols can be drawn on the writing surface.
With this known lettering device, in which the movement of the writing part relative to the drawing surface consists of a complicated combination of polar coordinates, not only the writing arm, but also the writing arm drive motor is located on the writing part which moves along the guide track. Thus, the writing part has considerable weight which must be moved back and forth along the guide track when drawing the symbols, thereby considerably restricting the writing speed, since at a high writing speed the mass being moved back and forth cannot be stopped quickly enough.
With a second known lettering device of this type (U.S. Pat. No. 4,197,550 or DE OS 27 24 855) the writing part attached to the drawing head contains two working cylinders which attach to the writing pen and whose length is adjustable in each case by a motor attached to a base plate at the end opposite the writing pen. These base plates which also support each of the associated drive motors are swivellingly attached to the drawing part.
Through controlled changes of the lengths of both working cylinders, by means of motors, the desired symbols can be drawn. However, the resulting work area is relatively small and, in addition, each particular adjustment in the length of the working cylinders for drawing any given symbol depends on which area of the working field is being drawn on. In order to at least partially eliminate the difficulty, an additional drive is provided in this second, known lettering device, wherein the entire writing part can be lengthwise adjusted relative to the writing field to be produced, so that this lettering device necessitates three motors and drives.
A further disadvantage with this second known lettering device comprises the fact that the base plate, supporting the ends of the working cylinders opposite the writing pen, also must support the associated motor for driving the working cylinders. As a result, this entire drive arrangement must be swiveled, along with the base plate, when the base plate is swiveled. This leads to the limitations, as set out hereinbefore, in the writing speed.
A third numerically controlled writing machine is also already known (DE - AS No. 12 95 858) in which the writing pen is movably attached to a traveler which moves along cartesian coordinates, which in addition supports a drive motor and a gear for moving the traveler in a coordinate direction. This traveler is located on a transverse support which can be moved in the direction of the other coordinate and on which, in addition to the traveler, a gear and a drive motor for moving the transverse support are attached.
With this known drawing machine not only lettering, but also numerically controlled drawings are produced and it is clear that the drawing speed has to be relatively low. On the one hand, the traveler with the various construction elements, and on the other hand, an even stronger transverse support supports the traveler, and further construction elements, must have a relatively large mass which is very difficult to accelerate and slow down.
Thus, it is the object of the invention to produce a lettering device which, as a result of the particularly light execution of the movable parts, allows high drawing speed.
To obtain this object, a lettering device of the type mentioned at the outset is produced according to the following invention, wherein the motors for moving the writing tip along cartesian coordinates are stationarily attached in relation to each other and the writing arm which holds the writing pen is attached so that it moves in the direction of one coordinate on a first guide track movable in a direction of the other coordinate by a first motor and is movable in the direction of the other coordinate on a second guide track movable in a direction of a coordinate by a second motor, wherein both guide tracks extend, in the area where they cross, preferably through a slide bearing body, to which the writing arm is attached.